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Genuine BMW
Coolant Level Sensor; Replacement Type
Located inverted at bottom of coolant expansion tank, headlight cleaning reservoir, and windshield washer fluid reservoir if so equipped. Up to 3 per car. This sensor will be replacement type, 17137553919, as superceded by BMW. Current version is blue and no longer has the hose sheathing.

thanks guys, on the home stretch, got done with cvv, putting alternator and OFHG back on, will be on the coolant level sensor soon. thanks starless for the cvv guide. i will curse the 1/4 turn hose for history, wasnt too bad with the OFHG out tho.

Some have a protective tube on the sensing area that should slide off easily before installation. It will be obvious when you get the part.
This blue plastic one pictured should be installed just like it is.

Thanks for the quick response. The blue plastic one does NOT have a protective sheath. Does the new sensor not require a protective sheath anymore? Or should I reuse the one from the old sensor. Thanks

the metal sensing wire is slightly exposed from the plastic in one area where the coolant can touch it.

They are Reed switches. No need for coolant to touch anything.

Here's my expansion tank on my M54 with the Reed switch in perspective with the magnet on the white float (we never did figure out what the black float does in the 2nd chamber):
- Behr radiator and Behr expansion (aka surge) tank autopsy (1) (2) & request for another autopsy (1)

Note the steel clip used by me to test whether the black "coin" was a magnet (it is):

Thanks everyone for all of the replies. I did the job last week and the sensor was probably the easiest part of the job. A half twist and it pulls right out. The new one pops right in. The plastic sheath is glued on there pretty good and won't come off. I was most concerned with nt having a sheath. The updated part has eliminated the need for the sheath and its a super easy swap out DIY.